Techniques for molding expandable resin beads vary considerably. The primary constituent for the expandable resinous beads is polystyrene. Many products have been formed for a variety of uses because of the high insulating properties of the expanded polystyrene beads and its flotation properties.
In the art of expanding prefoamed resinous beads, a cavity is defined by separable mold parts. Steam is injected into the cavity usually by the use of core vents which have relatively large openings to permit the introduction of steam into the beads contained in the cavity. The resultant expanded foam product has numerous surface markings of the core vent shape
In the art of molding products using expanded resinous materials, there is a significant demand for an expanded foam part which has a smooth surface. An example for this demand is in the requirements of "evaporative" casting as disclosed in Canadian Pat. No. 597,175. The technique includes embedding an expanded polystyrene part in molding sand and pouring into the part molten metal to burn and replace the area defined by the part with molten metal which cools to form the shape of the part. The metal reproduces every imperfection on the foam part surface, thereby necessitating a smooth part surface.
Canadian Pat. No. 1,102,067 discloses a method whereby a smooth surface for the expanded fused polystyrene bead structure is disclosed. The cavity wall is provided with a smooth permeable material, such as a sintercore metallic material. The expanded product, when removed from the mold, has an essentially smooth surface as provided for by the very small passages in the permeable walls of the sintercore metallic material.
Another technique, which results in a smooth surface product, is a "dry molding" technique which involves the use of small pinhole size apertures for injecting steam amongst the beads contained in the cavity to cause their expansion. In the dry molding technique, an elaborate chamber system is required having machined channels to isolate the steam which is used to heat the cavity walls from the steam which is injected amongst the contained resin beads. This arrangement, particularly when complex shapes are to be made, results in a very costly mold structure which cannot be readily modified and limits the location of the pinhole vents for injecting pressurized steam into the prefoamed beads.
An example a complex machined mold structure, which isolates the steam supply used, is disclosed in Bridges et al, U.S. Pat. No. 3,225,126.
In an effort to rid formed expanded styrofoam bead parts of surface imperfections, Yaita et al, U.S. Pat. No. 4,081,225, discloses a self-closing nozzle for delivering steam injected into the preheated, pre-expanded beads in a cavity mold. The difficulty with this system is that impurities in the steam supply and in the prefoamed bead supply may collect beneath the check valve arrangement and prevent full closure of the valve thereby resulting in the unwanted surface imperfections.
The mold structure, according to this invention, overcomes the above problems in providing an economical, readily manufactured mold for prefoamed resin beads.